This invention pertains generally to solid state microwave oscillators and in particular to an oscillator having low noise characteristics and incorporating a field effect transistor (FET) as the active element.
It is known in the art that solid state microwave oscillators may utilize diodes (such as GUNN or IMPATT diodes) or triodes (such as bipolar transistors or field effect transistors) as active elements. In many applications, it would be advantageous to use an FET (particularly a so-called gallium arsenide, or GaAs, FET) as the active element in a microwave oscillator. Unfortunately, however, it has heretofore not been possible to reduce the noise characteristics of GaAs FETS to acceptable levels for many applications.
As has been reported, in an article by O. Ishihara, T. Mori, H. Sawano and M. Nakatani entitled "A Highly Stabilized GaAs FET Oscillator Using a Dielectric Resonator Feedback Circuit in 9-14 GHz," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-28, No. 8, August 1980, pp. 817-824, frequency modulation (FM) noise of a GaAs FET oscillator may be reduced by providing a relatively high Q in the feedback circuit. However, the FM noise, although reduced, is still relatively high. Thus, for example, assuming that the FM noise of an X-band GaAs FET oscillator is typically -65 dBc/Hz at 10 KHz offset from the carrier, the use of a dielectric resonator in the feedback path could reduce this noise to -95 dBc/Hz at the same offset frequency.
As has been reported, in an article by B. T. Debney and J. S. Joshi entitled "A Theory of Noise in GaAs FET Microwave Oscillators and Its Experimental Verification,"IEEE Transactions on Electron Devices, Vol. ED-30, No. 7, July 1983, pp. 769-776, noise in such an oscillator may be attributable in large part to low frequency noise upconverted to the oscillator operating frequency by the nonlinear characteristics of the FET.